Added 'XGetErrorText()' to the 'int xerror()' function for more helpful error message.

Added 'XGetErrorText()' to the 'int xerror()' function for more helpful error message.

Based on:
 http://www.unix.com/man-page/all/3x/XSetErrorHandler/
 http://www.squarebox.co.uk/cgi-squarebox/manServer/XSetErrorHandler.3X11